There are many applications where a high precision phase angle determination is required for an AC siqnal of selected frequency. One such application is an ultrasonic Doppler system where the difference of successive phase angles of the received signal is indicative of both the direction of flow of blood at the point being scanned and of the velocity of such flow. The detected phase angle difference is utilized to control the color of the display in the selected area, the color being indicative of both the direction of blood flow, and its velocity. In certain applications, it is required that this information be provided with a high degree of precision.
In conventional systems utilized for detecting phase in this application, the incoming signal is split to produce two signals, one of which is mixed with a first signal at the frequency of the incoming signal and the other of which is mixed with a signal at the same frequency but 90 degrees out of phase with the first signal. The resulting signals are then digitized and the digitized values are applied as address inputs to a table-look-up ROM, the output from the ROM being the Arctan of the quotient of the generated signals. This Arctan value is equal to the desired phase.
With phase detectors of this type, higher degrees of precision are obtained by increasing the number of digits in the digitized signal, and thus in the address inputs to the table-look-up ROM. This means that for each digit increase in precision, the size of the ROM increases exponentially. High precision readings can therefore result in the requirement for a very large table-look-up ROM. Since the division function to obtain the Arctan is a relatively slow function as performed in a computer, obtaining the Arctan function in this way in order to obtain a phase value is not a viable alternative to the table-look up procedure in most applications.
A need therefore exists for a high precision phase detector which does not require the use of a large table-look-up memory while still providing rapid response.